I. Field of the Invention
This invention relates to a process for coloring surfaces of articles made of metals, especially those made of aluminum and anodizable aluminum alloys. More particularly, the invention relates to a process of this kind which avoids the need for the use of organic or inorganic pigments to achieve the desired coloring effect.
II. Discussion of the Prior Art
It is commonplace in the manufacturing industry to provide articles made of aluminum or aluminum alloy with colored surfaces in order to enhance the decorative appeal of such articles. For example, many beverage cans are made from aluminum alloys nowadays and the outer surfaces of such cans are commonly provided directly with a coating of colored paint or lacquer rather than a paper label or the like. Numerous other articles made out of aluminum are also provided with similar coatings for decorative or marketing purposes.
In addition to coloring aluminum surfaces with paint or lacquer, it is also known to provide such surfaces with a porous anodic film and to introduce an organic or inorganic coloring agent into the pores of the film. Organic pigments introduced into the pores in this way normally create a colored surface by the selective absorption of particular wavelengths of light. Inorganic pigments, such as small metal deposits, may produce a colored effect in the same way or, more usually, by effects including both light absorption and light scattering.
It is also possible to use metal deposits or discontinuous metal layers to create visible colors by light interference effects, for example as disclosed in our copending U.S. patent application Ser. No. 497,222 filed on Mar. 22, 1990. In such cases, light reflected from the metal deposits interferes with light reflected from the underlying metal surface and/or the outer anodic film surface to create interference effects. Non-dichroic or dichroic colors can be produced in this way and colors of good intensity from a broad spectrum can usually be generated.
The problems with the conventional coloring processes of the kinds mentioned above are that the coloring procedures can be difficult and expensive to operate and they necessarily introduce a foreign material, such as a paint or pigment, onto or into the surface of the aluminum article. Such materials must be removed when the aluminum article is recycled, thus complicating the recovery procedure. However, when attempts have been made to color aluminum surfaces using thin anodic films alone, the resulting coloring effects (even when obtained at all) are of very low intensity to the extent that they are not useful for commercial articles.